Among inorganic substances, calcium carbonate can be easily and inexpensively purchased anywhere, since its raw material, limestone, exists abundantly on the earth. Therefore, plastic thin film materials comprising a high amount of calcium carbonate have been recently proposed as environment-friendly paper, and practical use thereof has begun. (Patent Document 1)
The thin film materials are expected to be used similarly in the wide range of fields where paper and plastic are used, but currently their use is limited to a very narrow range due to quality issues.
On the other hand, conventional paper made of pulp is generally shipped after its qualities are made suitable for applications, for instance, by modifying papering methods or applying various chemicals such as a paper strength enhancer or process materials. However, since air permeability of paper is high and accordingly these chemicals, etc. easily permeate spaces between fibers of pulp, it was necessary to increase the application amount to at least equal to or higher than the permeate amount of the functional chemicals or process materials in order to secure even adhesion of the functional chemicals or process materials.
In the field of rapidly developing functional materials, the need for thin film is also high. While a film which has good air permeability has been developed by drawing of a film, on the other hand, there is a need for a thin film material which allows functional chemicals to adhere efficiently at a low cost.
While in the case where functional chemicals are made to adhere to a thin film material, it is important that the functional chemicals adhere firmly to the thin film material; specifically, as the polarity of polyethylene or polypropylene, etc. is low, the adhesional force is weak when these materials are coated or laminated. Therefore, a film substrate is often subjected to physical or chemical treatment to modify the surface, and then, subjected to processing treatment.
Surface modification for improving the adhesiveness of a film roughly depends upon the following two effects.
The first is a method wherein the surface is roughened mechanically or by means of corrosion so as to increase adhesional forces by an anchoring effect. The other is a method to form electrically polar functional groups on the surface by chemical reaction so as to increase adhesional forces.
However, addition of the afore-mentioned processing treatment step to manufacturing process of thin film materials incurs an increase in costs for thin film materials. Therefore, such addition is one of the causes to impede the realization of practical use of thin film materials.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H11-277623